> ## Documentation Index
> Fetch the complete documentation index at: https://docs.4mica.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Express

> Protect Express routes with 4Mica x402 payment middleware.

Use Express when you want the shortest server-side path to 4Mica. The `@4mica/x402` package ships Express middleware that handles HTTP 402 responses, guarantee verification, and settlement for protected routes.

## Prerequisites

* Node.js 18 or later.
* An Express app.
* A seller wallet address that receives payments.
* A buyer wallet with 4Mica collateral if you want to test the paid client.

## Install

<CodeGroup>
  ```bash npm theme={null}
  npm install express @4mica/x402 @x402/fetch viem
  ```

  ```bash pnpm theme={null}
  pnpm install express @4mica/x402 @x402/fetch viem
  ```

  ```bash yarn theme={null}
  yarn add express @4mica/x402 @x402/fetch viem
  ```

  ```bash bun theme={null}
  bun add express @4mica/x402 @x402/fetch viem
  ```
</CodeGroup>

## Create wallet

Create one wallet for the buyer client and one seller address for `payTo`. For wallet setup, see [Wallet](/core-concepts/wallet).

Keep the buyer private key in an environment variable:

```bash theme={null}
PRIVATE_KEY=0xYourPrivateKey
PAY_TO=0xYourSellerAddress
```

<Warning>
  Use the private key only for buyer-side signing. Use `PAY_TO` as the seller
  address advertised by protected routes.
</Warning>

## Deposit collateral

Deposit collateral for the buyer wallet before it calls the protected route. Use Base Sepolia (`eip155:84532`) while you test, then wait for the deposit to finalize.

To learn more about collateral setup, see [Deposits and Withdrawals](/core-concepts/deposits-and-withdrawals).

## Protect a route

```ts theme={null}
import express from "express";
import { paymentMiddlewareFromConfig } from "@4mica/x402/server/express";

const app = express();
app.use(express.json());

app.use(
  paymentMiddlewareFromConfig({
    "GET /premium-content": {
      accepts: {
        scheme: "4mica-credit",
        price: "$0.10",
        network: "eip155:84532",
        payTo: process.env.PAY_TO ?? "0xYourAddress",
      },
      description: "Access to premium content",
    },
  }),
);

app.get("/premium-content", (req, res) => {
  res.json({ message: "This is premium content behind a paywall" });
});

app.listen(3030, () => {
  console.log("Server running on http://localhost:3030");
});
```

## Support payment

```ts theme={null}
import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { FourMicaEvmScheme } from "@4mica/x402/client";
import { privateKeyToAccount } from "viem/accounts";

const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const scheme = await FourMicaEvmScheme.create(account);

const fetchWithPayment = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [{ network: "eip155:84532", client: scheme }],
});

const response = await fetchWithPayment("http://localhost:3030/premium-content");
const data = await response.json();

console.log(data);
```

## What happens

1. The buyer requests `/premium-content` without payment.
2. Express returns HTTP 402 with payment requirements.
3. The wrapper creates a unique request ID, signs a guarantee, and retries with a payment header.
4. The middleware verifies and settles the payment before your handler serves the response.